Generally speaking, it needs 33% expansion of a picture to the vertical and horizontal direction of screen to fill a picture having vertical and horizontal ratio of 16:9 on the cathode ray tube without a variation in the ratio when the vertical and horizontal ratio of a picture is 4:3. This is called with zoom mode, so as to materialize this zoom mode, and needs a circuit that is able to vary a vertical position of a picture.
Referring to the FIG. 2, this is a drawing for showing a state of a picture on zoom mode. As shown in the upper and lower portions of FIG. 2, in absence of the the circuit for varying a vertical portion of a picture, the picture positioned to the vertical direction is lost.
FIG. 1 is a conventional circuit diagram which is able to materialize the zoom mode for solving this problem.
This circuit for varying a vertical position of a picture comprises a horizontal and vertical oscillator part 1 for detecting a vertical and horizontal synchronization signals, and outputting a horizontal and vertical pulse from an inputted video signals, a vertical pulse delay part 2 for delaying the vertical pulse outputted from said the horizontal and vertical oscillator part 1 for the time of 265.+-.n-lines, a vertical driving circuit part 3 which makes a voltage amplification, a linearity amendment and a amplitude adjustment of saw tooth wave pulse according to a delayed vertical pulse and a vertical position signal, a vertical deflection amplification part 4 for outputting the amplified pulse to a vertical deflection coil (hereinafter referred to as V-DY) after amplifying a vertical a tooth pulse inputted from the vertical driving circuit part 3. A vertical position detector 5 for detecting a vertical position of a picture and providing a feedback for the detected signal to the vertical driving circuit part 3. A Micom 6 for controlling the circuit including the vertical pulse delay part 2 and the vertical driving circuit part 4 etc.
As shown in the FIGS. 1 and 2, the horizontal and vertical oscillator part 1 detects a horizontal and a vertical synchronizing signals inputted from an external video signal, thereby outputs the detected vertical synchronizing signals to the vertical pulse delay part 2. The vertical pulse delay part 2 delays the vertical pulse inputted from the horizontal and vertical oscillator 1 for the time of 262.5.+-.n-lines according to the control of the Micom 6 and inputs the delayed vertical pulse to the vertical driving circuit part 3.
The operation of the present invention will be described, with references to FIGS. 1 and 2.
Here, when the vertical pulse delay part 2 delays the vertical pulse being inputted for the time of 262.5+n, since a vertical synchronization of output video signal is delayed for the time of n-lines with respect to the input video signal, a picture to be displayed on the screen is raised by the time of n-lines in the vertical direction with respect to the normal position.
In contrast, when the vertical pulse delay part 2 delays the vertical pulse being inputted for the time of 262.5-n, since the vertical synchronization of output video signal becomes faster by the time of 262.5-n lines than the input video signal, a picture to be displayed on the screen is dropped down by the time of n-lines in the vertical direction with respect to the normal position. The vertical driving circuit 3 which receives the delayed vertical pulse outputted from the vertical pulse delay part 2 as above two cases, transforms the vertical pulse into a vertical saw tooth wave and inputs it to the vertical deflection amplification part 4. The vertical deflection amplification part 4 amplifies the inputted vertical a tooth wave pulse and transmits the amplified vertical a tooth wave pulse to the V-DY.
At this time, to adjust the vertical center point, a constant vertical center point direct voltage which is divided by the resistances R1, R2 in the vertical position detection part 5, is applied to the point A, and the voltage flowing through the V-DY with alternate current (hereinafter referred to as "AC") component is made a feedback to the vertical driving circuit 3 through condenser C1, resistance R3, condenser C2 in order.
The vertical driving circuit part 3 which receives a voltage with AC component being made a feedback from said the vertical position detecting part 5 through a negative terminal (-), makes an amendment of a vertical linearity, S-type or parabola.
In addition, the vertical center point voltage which is divided by the resistors R1, R2 in said the vertical position detecting part 5, is made to feedback to the vertical driving circuit part 3.
The vertical driving circuit part 3 which receives the vertical center point voltage from a feedback, amends a vertical position of a picture. However, this conventional circuit for varying a vertical position of a picture has a problem that a difference in the number of horizontal lines per each field is generated by a few lines in contrast with a standard signal.
Accordingly, when this conventional circuit generates a quasi vertical synchronizing signal delaying a signal for the time of 262.5.+-.n-lines as shown in FIG. 3B from the video signal in which a difference in the number of horizontal lines per each field occurs as shown in FIG. 3A, this circuit has a problem of interlace phenomenon that a picture vibrates in the vertical direction because of a difference in the number of lines between the virtual video signal and the field (F2 in FIG. 3A) which becomes a reference of the quasi vertical synchronizing signal.